1. Field of the Invention
The present application relates to apparatus and methods for assisting operators of vertical takeoff vehicles in landing operations within environments of low visual acuity. In particular although not exclusively the present application relates to a radar altimeter for assisting in landing operations of vertical takeoff vehicles.
2. Discussion of the Background Art
Many incidents and accidents involving the use of vertical takeoff vehicles occur frequently during takeoff and landing operations. Given the complex mechanics of the vehicles involved a wide array of variables can influence the success of these operations. Numerous accidents causing anywhere form minor damaged or at worst destruction of the vehicle have resulted from environmental factors such as terrain type, wing and rotor clearance etc.
One cause of accidents in landing operations is setting the vehicle down on steep or uneven terrain. From the air it can be difficult to sight imperfections in the landing area. Setting the craft down on steep or uneven surfaces can cause the craft to pitch and tip over. Another major factor in terms of the terrain type, which can affect landing and takeoff operations, is the composition of the landing/takeoff surface. For example in arid landing and takeoff environments sand, dust and other debris can be kicked up into the air by the down□draft created by the vehicle. This situation is known as a “brownout”. In essence the debris that is kicked up into the air creates a cloud which can completely obscure the operators view to the landing zone and surrounding landmarks.
In the case of vehicles such as a helicopter, the increased turbulence created by the rotors (rotor wash) can create eddy currents within the particulate cloud. The swirling mass of particles can lead the pilot to experience the vection illusion, a form of spatial disorientation where the helicopter appears to be turning when it is actually in a level hover. A pilot not using the flight instruments for reference may instinctively try to level the aircraft, resulting in an accident.
The brownout problem has been keenly noted by the US military. Since 1991, there have been over 230 cases of aircraft damage and/or injury due to unsuccessful take-offs or landings in arid environments. Although the majority of the incidents occur during landing, there have been a significant number of incidents occurring during takeoff. Of the more than 50 brownout incidents occasioning damage reported during the period of 2001-2007, 80 percent were during landings and 20 percent during takeoffs. On average the number of brownout incidents occasioning damage costs the US military US$100 million per year.
In addition to the brownout problem a large number of helicopter operations are carried out under the cover of darkness. Typically these operations require helicopters to minimise their presence by flying low and without lights. During such operations pilots become more dependent upon their instruments and limited information regarding terrain from night vision systems (if available).
One piece of equipment which can provide a pilot with valuable information in both brownout conditions and night time operations is a radar altimeter. The basic radar altimeter utilises a radar ranging system which measures the time delay of the signal reflected from the nearest object within a single wide beam illuminating the ground. This wide-beam is intended to monitor aircraft height even when in a bank or flying near steep slopes.
One example of a radar altimeter is shown in U.S. Pat. No. 5,047,779 to Hager which is capable of tracking at least two targets. The altimeter of Hager information relating to the first target is captured via a first set of radar antennas and stored before the altimeter switches to a track and store information of the second target via a second set of radar antennas. U.S. Pat. No. 6,750,807 also to Hager et al describes a similar scheme, but with a forward-looking scanning beam for obstacle warning. Both arrangements proposed in Hager patents simply provide range information to both targets and as such are generally useful in assisting a pilot with obstacle avoidance in flight. Neither of the altimeter Hager patents is capable of providing the pilot of any useful information regarding the tomography of the desired landing surface.
One approach to assisting pilots with obstacle avoidance in degraded visual conditions is discussed in U.S. Pat. No. 7,106,217 to Judge et al. The approach of Judge relies on merging data from a number of sensors to allow a display and fly-by-wire capability in poor visibility. While the system of Judge is capable of assisting a pilot with identifying the position of the aircraft relative to the select landing zone and any obstacles within range of the aircraft, it does not provide the pilot of any useful information regarding the topography of the desired landing surface.
Thus it would be advantageous to provide a system and method that would not only assist an operator of a vertical takeoff vehicle in identifying obstacles in flight and around a landing zone, but also provides the operator with information on the topography of the landing surface. It would also be advantageous to provide a system and method that is capable of producing accurate information regarding the topography of the landing surface in conditions of minimal visual acuity.